Handheld devices like cell phones and PDAs rely on a variety of components to perform power management. Maximizing the efficiency of these components, both individually and as a whole, is becoming increasingly important as power consumption climbs (Fig. 1). For example, as cell phones take on new functions like still imaging, video, and Internet access, and move from monochrome to color displays, the drain on the battery rises, decreasing runtimes.
Incremental improvements in the Li-ion batteries used to power handheld equipment have helped to offset some of the increases in power dissipation. But these gains alone aren't enough to maintain the desired runtimes. Consequently, ongoing efforts are made to improve the various power-management components that affect runtime. Battery-management ICs, such as the charger and fuel gauge, are among the components being honed to serve emerging power-hungry applications.
Li-ion battery-management ICs fall into two broad categories. There are those developed for single- and dual-cell applications, which are generally the handheld products like cell phones, PDAs, and digital cameras. The other category involves multicell applications with three or more Li-ion cells. The most familiar example is the laptop, where as many as nine cylindrical Li-ion cells might be found. The requirements for these applications are very different due to the variations in battery capacities, discharge rates, space constraints, and cost.
In general, the laptops require and can justify the added cost of more-efficient charging circuits and more-accurate fuel gauges. However, with handhelds becoming more sophisticated and in need of power, the gap in power-management requirements separating handhelds from multicell applications is closing. Consequently, semiconductor manufacturers are striving to recraft their battery-management components and design solutions. In the process, they're finding ways to migrate some of the battery-management techniques applied to laptops to the handheld devices. In general, they seek to make the new battery-management solutions smaller, more integrated, easier to implement, and less costly. While the focus here is on standalone battery charging components, there also are ongoing efforts to integrate battery charging functions within large power-management ASICs.
Although so many of the battery-management components for handhelds are aimed at high-volume consumer applications, relatively few designers are working on cell phones, PDAs, and the like. Nevertheless, battery-management ICs developed for these products stand to benefit those designers working on a much wider range of products that rely on one or two Li-ion cells (including the Li-polymer types) for power.
With the cost of Li-ion cells tumbling, these high-energy density batteries will take hold in more equipment designs. CD and MP3 players, electric shavers, GPS units, medical monitoring devices, and infusion pumps are among the newer single- and dual-cell Li-ion applications.
In handhelds, battery-charging chips have been developed mainly with small size and low cost in mind rather than efficiency. As a result, the single- and dual-cell battery chargers are more often linears than switching ICs. When first introduced, these linear charge chips required three external discretes—a pass transistor, a reverse blocking diode, and a current-sense resistor. These elements are now integrated within many of the single/dual-cell chargers.
Yet, vendors continue to expand the functionality of these ICs by adding safety, monitoring, and control features. Chip vendors are also developing more compact solutions by introducing new charger ICs in the more space-efficient quad flatpack no leads (QFNs) rather than the larger TSOPs. The QFNs are thermally enhanced with a die attach pad that helps remove heat from the packages, making them suitable for integration of pass transistors.
This feature aids designers in implementing fast charging. Nevertheless, the charger is often embedded in the handheld product, where the heat dissipated by the charger can create problems. One response has been to implement pulse charging, a technique that pushes the power dissipation from the charger's IC back to the ac adapter (usually a wall plug with 5-V output). Although this technique is somewhat controversial (battery vendors don't recommend it), chip vendors claim that it doesn't harm the battery. Another trend in single/dual-cell battery charging is the development of charger ICs crafted to run off the popular USB port.
Meanwhile, many of the same semiconductor vendors are busy developing fuel gauges for the handhelds. These fuel gauges will make it possible to measure battery capacity with much higher accuracy than what most handhelds currently achieve with voltage-based battery monitoring. The improved accuracy will ensure that batteries are charged more fully and permit more accurate detection of low-battery conditions. These improvements will combine to extend battery runtimes in single- and dual-cell applications.
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